The air-gap of an electrical machine is the distance between a movable, often rotatable, part of the machine and a static part of the machine. To prevent damages the air-gap has to be designed properly to avoid a mechanical contact between the two parts.
The air-gap has to be designed in a way, that even tolerances of used components within the two parts do not influence the function of the machine.
Even a rotational force or a gravity force or a thermal expansion is acting on the components of the machine, which needs to be considered by designing in the air-gap.
Additionally it is preferred, to achieve a uniform, constant air-gap to avoid a synchronic load cycling of the equipment.
A uniform, constant air-gap also leads to a constant shape of the rotating part of the machine, so an unbalanced rotating mass is avoided.
Right now the air-gap is a result of the manufactured components e.g. rotor/stator and will make its appearance, while the machine is assembled. The air-gap design accounts for the problems stated above, so the air-gap-design results in a maximum possible air-gap with its negative effect on power and efficiency.